A big part of the reason that many of the biggest sites websites on the net now are made in Php, is because it was the most viable and popular option 5 years ago. In that time both Ruby and Python have made leaps of advances in using their technology for web development. Now both languages are faster than Php, and in general the popular frameworks in this languages also preach better practices.

I worked with Php for a couple of years and I can tell you that I absolutely thought the language was a melting pot of some great ideas with a huge bunch of WTF form the very beginning. In my opinion Php as a language is archaic and the only reason to use it now is because it makes sense for your team (because that's what your team has experience with). That being said, a good developer gets great stuff out the door regardless of what language they use to do so. Hell one of best developers I know is a Php junkie - with whom I always ended up having a slight language war with. The funny thing is that even tough he was the strongest Php advocate I know, he is now using Python for everything and agrees that it's a much better language.

Now I work almost exclusively with Ruby for webdevelopment (and also for a big part of my server scripts now that I mention it) because it just makes sense. The code is a lot more maintainable and scalable, though this is because it's generally the case that out of a batch of Ruby developers you will probably get more 'good' developers than out of a batch of Php developers with the same amount of people. It's my experience that its easy to get 50 Php developers, out of which only a couple are actually good ones, while if you get 10 Ruby devs, at least half are competent.

This rant is getting long so I'll just leave you with a thought. If the only developers you can get right now are Php devs, it makes sense to do your project in Php; however, if it's possible for you to get your hands on some Ruby or Python developers, you really can't go wrong by choosing either over Php, specially since doing so will generally have the benefits of a faster, better, and more maintainable codebase.